This invention relates generally to the measurement of physiological pressure, and more particularly to an automated closed venous pressure measuring system.
While a system in accordance with the invention is capable of measuring physiological pressure such as arterial and venous blood pressures as well as intracranial, spinal, gastrointestinal, esophageal and intrauterine pressures, for purposes of explanation it shall be described mainly in the context of venous pressure measurement.
During many surgical procedures as well as in post-operative stages, it is now the usual practice to measure and record venous pressure. Venous pressure trends are a significant consideration in both diagnosis and prognosis. Moreover, such measurement is also important with respect to cardiovascular diseases; for an upward trend is one of the first signs of congestive heart failure.
The most commonly-used venous pressure measuring instrument is of the manometer type and includes a three-way stopcock and an intravenous needle to provide an arm puncture. The longitudinal passage in the stopcock connects the needle to a syringe, while its vertical outlet is coupled to the graduated manometer tube. In operation, blood is first drawn into the syringe which contains a sterile saline solution. After manipulation of the stopcock valve, the manometer tube is filled by expressing the mixture from the syringe. Thereafter, communication is restored between the manometer tube and the vein. This manometer, stop-cock combination may be used as part of an intravenous infusion apparatus, in which case the longitudinal passage in the stopcock connects the needle to the fluid supply, and the vertical outlet is coupled to the graduated manometer tube. During normal feeding, the fluid supply is connected to the needle, and the manometer tube is isolated. To read the pressure, the stopcock is first turned until the fluid supply is connected to the manometer (to flush the manometer), after which the stopcock is turned to connect the manometer to the needle. After the pressure is read, the needle is re-connected to the fluid supply.
The level to which liquid in the manometer falls upon reaching equilibrium is read directly from the tube scale, thereby providing a venous pressure reading. It will be recognized that the operating procedure for the manometer is somewhat complicated. Also, to obtain a correct reading, care must be exercised; and for this purpose the base of the manometer must be properly aligned with an appropriate point on the patient's body.
Venous pressure can vary through a relatively wide range: from a low of zero (or even negative values) relative to atmospheric pressure, to a high of 25 or 30 cm of water above atmospheric pressure. As a consequence, the conventional venous pressure instrument requires a fairly long manometer tube as well as associated apparatus that must be carefully assembled and properly supported. Because the upper end of the manometer tube is open to the atmosphere, the fluid is exposed and may be subject to bacterial contamination that is transmittable to the patient.
The intravenous (IV) infusion of various types of fluids to patients is a routine hospital procedure. In a standard IV system, a plastic bottle filled with a fluid such as glucose, serum or plasma, is supported at an elevated position to provide gravity flow through a flexible line whose end is coupled to a needle injected into a vein of the patient. In practice, there are many situations in which it is desirable to be able to infuse fluid into a patient or to take a venous pressure reading, using the same line for this purpose.
My prior patent (Miller), U.S. Pat. No. 3,980,082 (1976) discloses a venous pressure measuring instrument which is interposable in an intravenous infusion line or is usable separately therefrom to effect venous pressure measurement without exposing any of the fluid to the atmosphere, thereby avoiding the possibility of contamination. This venous pressure measuring instrument is interposed in a flexible infusion line leading to a patient, the downstream portion of the line functioning effectively as a manometer column, thereby dispensing with a supported manometer tube and the need to align the base of a remote manometer tube with a point on the patient's body.
The instrument disclosed in my prior patent is constituted by a transparent chamber interposed between an upstream line extending to the bottle containing the infusion fluid and a downstream line leading to a cannula. Visibly disposed within the chamber is a collapsible bulb that is vented to the atmosphere, the bulb being erect when the fluid pressure exerted on its exterior is not greater than atmospheric, the bulb collapsing when the fluid pressure is greater than atmospheric. Because the vent communicates only with the interior of the bulb, the fluid in the system is sealed from the atmosphere and contamination thereof is avoided.
In the infusion mode, fluid derived from the bottle and going to the patient passes through the chamber, and the resultant fluid pressure exerted in the bulb causes it to collapse. In the venous pressure mode, the upstream line is clamped to block flow from the bottle, as a result of which the fluid pressure then exerted on the bulb is a function both of venous pressure and the height of the chamber relative to the heart--the higher the chamber, the lower the fluid pressure.
By elevating the chamber to a point at which the fluid pressure exerted on the bulb is equal to atmospheric pressure, a balance or reference level is established, this balance level being indicated by the sudden erection of the bulb. Venous pressure is then determined by measuring the height of the chamber at the balance level relative to the heart, this measurement being made on a scale formed on the downstream line.
In another embodiment, the patented instrument is independent of an IV system, in which instance the chamber has no inlet but only a flexible line connection to the patient. The balance level is attained when the elevation of the chamber is such that at the interface of the air compressed in said line and the fluid derived from a body cavity, the pressures are equal, the body pressure being determined by measuring the height between the interface at the balance level and the body point.
Though a venous pressure measuring system of the type disclosed in my prior patent has distinct advantages over conventional instruments of the manometer type, it has certain practical drawbacks when used in hospitals. Because of the prevailing shortage of staff nurses, available personnel lack the time and patience necessary to take venous pressure readings which require up and down movement and observation of the degree of bulb collapse.
The need exists, therefore, for a venous pressure measuring system which is fool-proof and easy to operate, and which affords a direct readout, digital or otherwise, so that no particular care is demanded of the operator in order to obtain accurate and reliable measurements.
One prior attempt to provide an easily-operated system for monitoring physiological pressure is disclosed in the Minior et al. U.S. Pat. No. 4,185,641 assigned to the Hewlett-Packard Company and in the bulletins published by this company in connection with its model 1280C and model 1290A physiological pressure transducers.
In the Hewlett-Packard pressure measuring system, the fluid passing through a line leading to the patient flows into the hollow of a pressure dome closed by a flexible membrane, the membrane being distended as a function of fluid pressure. This membrane is maintained in intimate contact with the diaphragm of a pressure transducer to produce an electrical signal as a function of physiological pressure. As pressure varies, the membrane deflects the diaphragm accordingly and causes corresponding changes in the signal which provides a pressure indication.
The accuracy and reliability of the Hewlett-Packard instrument depends on the physical coupling between the dome membrane and the transducer diaphragm in intimate contact therewith. As pointed out in the Minior et al. patent, if the pressure dome is attached too lightly to the transducers, this will produce an offset in the fluid pressure signal; and if the dome is attached too loosely, the fluid pressure will not be fully transmitted to the transducer, again causing an erroneous reading.